1. Field of the Invention
The present invention relates to a light control circuit, and in particular, to a light control circuit suitable for a brightness control of a lighting device that illuminates a liquid crystal device.
2. Description of the Related Art
In general, in a liquid crystal device used as an image display unit of a mobile phone or the like, a lighting device that illuminates a liquid crystal device is provided and the brightness of a screen is controlled corresponding to the surrounding brightness.
Various kinds of light control devices for the brightness control have been proposed (for example, see JP-A-10-191467 and JP-A-2003-68480).
A known light control device disclosed in JP-A-10-191467 is formed as shown in FIG. 5, for example. Referring to FIG. 5, an illuminance level detected by an illuminance sensor 21 formed using a phototransistor or the like is output, as a change of a voltage through a resistor 22 for current/voltage conversion, to an A/D converter 24. Then, a voltage value, which is an analog value, input to the A/D converter 24 is converted into a digital value and output to a control circuit 25. The control circuit 25 calculates the brightness of a lighting device 27 corresponding to an input voltage value and calculates the brightness of a light-emitting diode 28, which is to emit light in the lighting device 27, and converts the calculated brightness into, for example, a PWM signal and outputs the converted signal to a driving circuit 26. A plurality of light-emitting diodes (only one light-emitting diode is illustrated in FIG. 5) that form the lighting device 27 are connected to the driving circuit 26. In addition, the driving circuit 26 outputs a driving signal corresponding to the brightness of the light-emitting diodes to emit light, which is output from the control circuit 25, in order to cause the light-emitting diodes to emit light. Moreover, when the illuminance level detected by the illuminance sensor 21 changes, each constituent part operates corresponding to the change, such that the light-emitting diodes emit light in the brightness corresponding to the illuminance level.
However, this know device has the following problems. Since an output of the illuminance sensor 21 is A/D converted and is read in a digital manner, the brightness of the lighting device changes in a stepwise manner according to the brightness of surrounding light, as shown in FIG. 6. As a result, the eyes feel very uncomfortable.
Further, this device also has a disadvantage in that flickering occurs near the threshold of the A/D converter 24, for example, in an ‘a’ portion of FIG. 6 due to the change of the brightness of the lighting device. In order to solve the problem, if a gray-scale level in the light control is increased, the flickering is reduced. In this case, however, the size of a circuit increases. As a result, the cost increases.
Furthermore, in the case when a period of a PWM output to the lighting device is long, flickering has occurred. In order to solve the problem, if the period of the PWM is made short, the flickering is reduced. In this case, however, a clock period should be short. As a result, there has been a problem in that the power consumption of a control circuit increases due to the high-speed clock. In addition, in the case when the PWM period is made short, it has been difficult to minutely control the duty.
Moreover, the surrounding brightness ranges from several lux in a dark room to several hundreds and thousands lux in the case of sunlight. That is, the dynamic range is very large. Accordingly, in the case when the surrounding brightness is controlled in the digital manner in the same manner as in the related art, a large dynamic range is required for a read circuit of an illuminance sensor and a driving circuit of a lighting device. As a result, there has been a problem in that the circuit size increases.
In addition, a liquid crystal device includes a reflective LCD or a transflective LCD in which reflection weighs and a transmissive LCD or a transflective LCD in which transmission weighs that is opposite to the reflective LCD or the transflective LCD in which reflection weighs. A lighting device that needs the reflective LCD or the transflective LCD in which reflection weighs is a lighting device that emits a large amount of light in a dark place and emits a small amount of light in a bright place. A lighting device that needs the transmissive LCD or the transflective LCD in which transmission weighs is a lighting device that emits a small amount of light in a dark place and emits a large amount of light in a bright place. In the related art, there has been no light control device capable of properly controlling the amount of emitted light in an analog manner in correspondence with the both cases described above.